It is definitely understandable to ask what this poem is about, because there is a lot going on here. Let’s take things in stages, to show some of the complexities of this poem.
First, a few historical facts. Hayden was a major African American poet. Wheatley was the first major African American poet. In writing a letter from her, Hayden first shows respect for Wheatley, paying her the homage she deserves and drawing attention back to her.
Next, Wheatley wrote a number of poems in letter form, addressing major historical figures like George Washington. In writing a (fictional) letter from Wheatley, Hayden again bows in her direction, modeling his work on her work.
A third layer is the profound irony of this poem. In it, Hayden has Wheatley comment on sailing across the ocean and how easy it was. This is compared to her earlier voyage, which was crossing the ocean to be a slave. He also has her comment on American society, the racial divide in it, and the gap between the Christianity they claim and the racism they practice.